Since the advent of the laser in 1960, efforts to scale particular laser oscillators have failed to realise high power outputs over apertures in excess of about 50 cms in diameter. The reasons for this limitation are complex but can be summarised as being either due to the inability to excite increasingly large laser gain medium volumes or due to the onset of parasitic oscillations within the expanded laser oscillator cavity which depletes the stored energy that would otherwise be used to generate the laser beam. Adding the output of individual laser oscillators to produce scalable laser output beams does not improve matters unless the scaled output laser output beams does not improve matters unless the scaled output laser beam is actually a single laser beam in its own right. The present invention achieves this requirement by locking together the laser beam generating properties of a large number of laser oscillators so that their combined outputs behave like a single beam output. Two major problems have to be overcome, namely, that the polarisation of the sub-oscillators have to be aligned with respect to each other and each of the said sub-oscillators is sufficiently close to its neighbour to ensure that phase-locking occurs.
In the present invention the alignment of the polarisation of the output of the various sub-oscillators is achieved via the use of appropriately stacked, single mode, single polarisation optical fibres whilst the necessary close packing of the oscillator output apertures is achieved via the packing together, coherently, of the optically polished ends of each of the fibres, into a composite aperture which behaves as a single beam laser oscillator aperture. The lasing medium used to generate the laser radiation within the laser sub-oscillators can either be electrically or optically excited and are inserted into the individual single mode, single polarisation optical fibres and connected to them via appropriate optical components for example, lenses, and single mode optical fibre connectors. If the lasing medium is in the form of a semiconductor, then the excitation of said medium is via direct electrical current. On the other hand, if the lasing medium is in the form of doped optical fibre or doped crystalline segment, then the said laser medium can be optically excited via arrays of photo emitting diodes or miniature flashtubes. If the lasing medium is in the form of a gaseous medium within an appropriate container, then the excitation means can also be in the form of direct current.
For a laser oscillator to be scalable, it is necessary for the same operating conditions to apply for a given small cross-sectional area of the laser structure as applies for cross-sectional area many orders of magnitude larger. For example, the operating properties of a one millimeter square area of the output aperture must behave in the same manner as an area of many tens of square meters if required. In the present invention, scalability depends only on the number of individual sub-oscillators that can be added together.
This can be achieved in two ways, firstly, by building tapes of optical fibres and stacking them together to form a coherently packed array or by winding the optical fibre onto a reel and cutting the wound fibre, again achieving a coherently packed fibre array. By grouping the fibres together so that each group can be excited from a single laser amplifying medium the output for said group can be phased locked. By grouping the sub-groups together to form a super group of fibres, the invention can be scaled indefinitely whilst maintaining a phased locked output. In other words individual fibre laser oscillators can be grouped together to form a phased-locked single beam output from a composite laser oscillator whilst such oscillators can also be grouped together to form a super group of composite oscillators. Alternatively, the grouping can be dispensed with entirely and the fully scaled composite oscillator be formed entirely from the ungrouped fibre oscillators. However, the grouping of the individual fibre oscillators during the scaling process allows for simpler addition since each group can be fully tested before inclusion into a super group as the aperture is scaled up.
In general, the closer together the individual fibre oscillators are in the final optically polished apertures, the more effective will be the phase locking process. This means that the cladding of the optical fibres used need not be of such large dimensions as in the case for the optical fibres used for optical communications. In general, the thinner the cladding thickness the more the loss in the fibre--which in the case of the present invention can contribute to interaction between the laser light transmitted within the fibres necessary to ensure excellent phase-locking.
The optically polished end faces of the individual fibres can either be achieved by separately polishing each fibre or by polishing the coherently packed end faces as a whole. If the fibres are polished individually, then their ends have to be positioned so as to achieve a final output aperture which is optically polished.
In order to operate the composite laser oscillator it is necessary to set up two mirrors, one each end of the oscillator cavity. The simplest way of doing this is to deposit a mirror onto the optically polished end faces of the coherently packed fibres. Another way of utilising the required mirrors is to deposit the reflecting surface onto a separate substrate which is also optically polished and then press it against the end surfaces of the fibre faces.
When all of the laser media are excited together, all of the fibres constituting the end faces of the composite oscillator emit in a phase-locked manner. However, this mode of operation, particularly when the number of fibres used are not large, leads to high non-uniformity of the laser output beam. To cure this defect it is necessary to modify the spacing between the various rows of fibres forming the output aperture. This can be achieved in the present invention by simply not activating various rows in fibres.
The ability to selectively switch individual fibres or groups of fibres allows the invention not only to have an optimised output beam but also provides the means for generating high definition images on the output apertures. For example, if the letter "O" were to be formed, then either all of the fibres in the output aperture corresponding to the letter "O" would not be activated whilst all other fibres would be activated then a high definition "O" is generated. Alternatively, all of the optical fibres corresponding to the letter "O" could be activated and all the others left unexcited. To generate a high definition image on the output aperture of the composite oscillator, a computer control of the excitation of the laser medium arrays is necessary. If a full colour presentation is required then each of the basic transmitter sites must include three fibre transmitters of blue, green and red respectively. In this way the invention converts into a high definition picture transmitter, capable of projecting a high intensity laser beam image. Alternatively, the invention can be used as a laser marker either in a divergent or convergent beam mode.
The high definition image generation on the output aperture of the composite laser oscillator results from the high packing densities achieved, for example a ten micron diameter fibre would provide an array of 1,000.times.1,000 per cm.sup.-2 of the output aperture. The input signal to the control computer could be from a TV station so that the output of the laser oscillator would be in the form of TV images. Since the output beam intensity would be relatively high, up to several kilojoules, the composite oscillator with computer control of the firing sequences of the laser medium arrays, would allow for the laser making of any material surface via the burning of said TV or other images directly onto said surfaces. The invention can be operated either in the continuous or pulsed mode since all of the heatable regions of the device, in particular the excited laser media, are well spread out and could be cooled if it was necessary to do so.